In ink-jet technology, image quality of high-resolution images can be a function of both the ink-jet ink used to produce an image and the print medium upon which the image is printed. In ink-jet printing, droplets of ink are ejected from a printhead in response to electrical signals generated by a microprocessor and are deposited on a print medium, such as paper or polymeric substrates, to form the desired image.
The use of digital image-forming apparatus such as, for example, thermal ink-jet printers, large-format plotters, piezo-electric printers, large form plotters, laser printers, silver halide grade photo imaging apparatus, and others has grown in recent years. The growth may be attributed to substantial improvements in print resolution and overall print quality coupled with appreciable reduction in cost, and ease of use. Today's image-forming apparatus offer acceptable print quality for many commercial, business and household applications at costs lower than those offered in the past.
Perceived color quality can be characterized using any one of several color space systems, such as CIELAB or Munsell, as is well known in the art. With respect to Munsell color space, a given color is defined using three terms, namely Hue, Value, and Chroma. With respect to CIELAB color space, a color is defined using three terms L*, a*, and b*. With this system, L* defines the lightness of a color, and it ranges from 0 to 100 (with 100 being white). Additionally, the terms a* and b*, together, define the hue, where a* ranges from a negative number (green) to a positive number (red), and b* ranges from a negative number (blue) to a positive number (yellow). Additional terms such as h* (hue angle) and C* (chroma) are used to further describe a given color, as is known to those skilled in the art.
In general, a successful ink for color ink-jet printing must have the following properties: good crusting resistance, good stability, the proper viscosity, the proper surface tension, good color-to-color bleed alleviation, rapid dry time, no negative reaction with the vehicle, consumer-safety, and low strike-through. When placed into a thermal ink-jet system, the ink set must also be kogation-resistant. However, a single ink-jet colorant and/or ink, which has good chroma, gamut, hue angle, and environmental robustness (e.g., air-fastness, light-fastness, water-fastness) is not always optimal for use with other colorants and/or ingredients of that ink, or other inks with which it is used in combination. In other words, not only does an individual colorant and/or ink (e.g., cyan, magenta, or yellow ink), have to independently have acceptable color qualities, but it should also work well when used as part of a dye and/or ink set. Accordingly, it would be desirable to provide colorant and inks for use in ink-jet printing, which have improved print quality, reliability performance, and environmental robustness, when used individually as well as in combination with other dyes and inks.
When a high-resolution image is printed, major issues arise, namely, color quality and image permanence, i.e., how long the quality of the image will last. With respect to much of the print media currently on the market, printed images commonly exhibit undesirable characteristics relating to image permanence. One such undesirable characteristic is the gradual dye-fade that is sometimes observed when dye-based ink-jet inks are printed on porous media. Such fade is caused by air and, more particularly, by small amounts of ozone in the air. It appears that, over time, ozone reacts with many dyes commonly used in ink-jet inks, causing them to break down and to lose or diminish their intended color properties. It has been observed that when ozone reacts with ink-jet ink dyes, the intended color properties of a given dye may shift to another wavelength value along the visible spectrum. This effect causes a gradual change in the perceived colors of the printed image from what was originally intended, i.e., colorshift. For instance, “red shift” is caused by oxidation of cyan dyes. Existing techniques for mitigating red shift include protecting prints from exposure to the atmosphere, such as by displaying them under glass. Dye-fade and colorshift is more of a problem with certain dyes than with others. Cyan dyes tend to be affected to a greater extent by the presence of ozone in the air than do other dyes.
Some of the drawbacks associated with the combination of existing ink-jet inks and print media include bronzing on swellable photo media, haze and humid hue shift on porous photo media, and bleeding of colors, among others. “Bronzing” refers to a lustrous sheen that appears on a printed sample in reflected light, typically appearing as reddish-brown color. It tends to be associated with cyan dyes. One way to mitigate bronzing has been to raise the pH of the ink. Increasing the pH of the ink has its own drawbacks, as the resulting ink may degrade the printhead. Other chemical reactions which detract from the desired color quality can take place in the ink composition during storage. An ink-jet ink composition should be stable and capable of producing the desired color after several months in storage, and for some period of time after first use and exposure to air. Porous photo media problems that occur include: haze and humid hue shift on porous photo media, and bleeding of colors, among others.
“Humid hue shift” refers to the tendency of the ink and/or the colorant(s) present in the ink to migrate or spread undesirably into unprinted or printed adjacent areas of the media substrate. Humid hue shift typically occurs prior to the printed inks fully drying on a substrate or after the inks are printed when exposed to humid conditions. “Print quality” generally refers to the level of defects including, but not limited to, such as bronzing, humid hue shift and/or bleed of ink-jet printed marks, particularly with colored inks.
Various buffering agents or pH adjusting agents are often used in ink-jet ink compositions, as even a slight change of pH can cause some colorants (e.g., dyes) to precipitate. Typical buffering agents include such pH control solutions as Trizma Base, available from Sigma-Aldrich Corp. (Milwaukee, Wis.); 4-morpholine ethane sulfonic acid (“MES”); 4-morpholinepropanesulfonic acid (“MOPS”); and beta-hydroxy-4-morpholinepropane-sulfonic acid (“MOPSO”); hydroxides of alkali metals and amines, such as lithium hydroxide, sodium hydroxide, potassium hydroxide; citric acid, nitric acid, hydrochloric acid, acetic acid, sulfuric acid; amines such as triethanolamine, diethanolamine, and dimethyl-ethanolamine; and other basic or acidic components. If used, buffering agents, typically comprise up to about 10 wt % of the ink-jet ink composition.
Various salts are often used in ink-jet ink compositions, to control bleed, especially for printing on plain paper media. The presence of salts can cause some colorants (e.g., dyes) to precipitate. Some of the drawbacks associated with the combined use of salts in ink-jet inks and print media include: bronzing on swellable photo media, haze and humid hue shift on porous photo media, among others. Some of the drawbacks associated with the combination of salt containing inks and ink-jet pens include: decreased pen reliability, decreased decap time, and decreased environmental robustness of the pen. Typical bleed controlling agents include multivalent salts such as calcium nitrate, calcium chloride, calcium acetate, magnesium nitrate, magnesium chloride, magnesium acetate, and combinations thereof. If used, bleed control agents typically comprise up to about 5 wt % of the ink-jet ink composition.